The invention relates to a method and a device for controlling the working points of a series-resonant-circuit inverter.
Inductive heating is frequently used to transfer power to moving heating material. The operation is fairly simple. An alternating magnetic field produces eddy currents in electrically conductive material, and the electric losses in the heating material cause it to be heated. To produce the alternating magnetic field, an inductor is necessary which is fed by an alternating source. The transferred effective power is dependent upon the frequency of excitation and the exact load conditions (of the inductor together with the heating material). The frequency range is selected not only according to feasibility, but also according to the particular application. Because of the current displacement effect, also known as a "Heaviside" or "proximity" effect, higher frequencies cause the heating material surface to be heated. Lower frequencies have a larger penetration depth and thus heat the volume of the material. One application is the heating of rotating rollers in the calendar of paper manufacturing machines.
Converters are used as voltage supplies for such a system, and the term generally encompasses both rectifiers and inverters. For high frequencies, valve inverters are often used. The efficiency of this design is low due to the valve losses (.eta..apprxeq.70%). High-capacity semiconductors are a useful alternative. When properly triggered, they demonstrate low switching and forward power losses. In order to prevent an overload of the circuit elements, however, specific operating conditions must be adhered to. The principal task of open-loop and closed-loop control electronics is, on the one hand, the guarantee of a specific operating range and, on the other hand, the control of one or several performance quantities.
Series-resonant systems are often fed by a converter in order to remove load from the circuit elements. When such a series-resonant-circuit inverter is triggered with frequencies within the range of the self-resonance point, only small currents or voltages occur in the switching instant. Thus, the switching losses are considerably reduced. Departing from the defined operating range leads to a considerable increase in the losses. In some situations, this may destroy the switches.
In the paper, Senderohre, Thyristor oder Mosfet im Frequenz-wandler fur das induktive Randschichtharten und ihr Einflu.beta. auf die Anwendungstechnik [Transmitter Valves, Thyristors or MOSFETs (metal-oxide-semiconductor field effect transistors) in a Frequency Converter for Inductive Edge-Layer Hardening and their Effect on Application Engineering], in the German periodical "Elektrowarme International" [Electroheating International], 47 (1989), B4, August, pp. B192-B201, different frequency converter designs are analyzed with respect to their advantages and disadvantages as power sources for inductive edge-layer hardening. The paper also examines electric and electronic phenomena in conjunction with industrial-process and application engineering. The efficiency of the frequency conversion is highest when MOSFETs and thyristors are used. Values greater than 90% may be easily achieved. As a rule, a conventional valve generator survives an inductor failure without suffering any consequential damage. Generally, an anti-resonant circuit inverter with symmetrical thyristors reacts in a problem-free manner to such a breakdown. On the other hand, in the case of converters with transistors, an inductor failure can cause semiconductor components to break down.
The paper, Resonanzumrichter im Mittelfrequenzbereich [Resonance Converters in the Middle-Frequency Range], in the German periodical "etz", Volume 110 (1990), Issue 18, pp. 948-953, introduces, inter alia, series-resonant-circuit inverters and anti-resonant-circuit inverters. In the case of the series-resonant-circuit inverter, a rectangular characteristic curve results as an exciting voltage u, also called an excitation voltage, over the resonant circuit. The frequency of the exciting rectangular square wave oscillation can lie above or below the resonant frequency of the resonant circuit. If the resonant-circuit inverter is operated with non-disconnectible power semiconductors, then an operating frequency must be selected below the resonant frequency. If disconnectible power semiconductors are available, then the series resonant circuit can also be excited above the resonant frequency. An operating point within this range is advantageous, since the current-breaking performance of the diodes is then no longer significant. As a result of the reduction in switching losses, the operating frequency can be raised up to the range of a few hundred kHz. The current in the power semiconductor then exhibits the shape of a half-wave, which begins during the making operation with a negative instantaneous value and ends during the breaking operation with a positive value.
Shifts in the self-resonance point caused by parameter changes in the resonant circuit must be detected by the closed-loop control and treated appropriately. Such fluctuations occur to a large extent during inductive heating. They result, on the one hand, from the temperature dependence of the ohmic resistance of the material and, on the other hand, from the changes in the magnetic properties. For example, the relative permeability of ferromagnetic materials varies by factors of ten when the Curie temperature is exceeded. The material then behaves diamagnetically. This change has a direct effect on the reactance of the heating material, and thus on the impedance across the inductor terminals. In some applications, short-circuiting of the inductor also occurs.
In steady-state operation, the closed-loop control of a series-resonant-circuit inverter is supposed to adjust the performance capacity of the inductor to the setpoint values. Different quantities can be drawn upon to determine power output capacity, such as incoming power, resonant-circuit power, temperature of the workpiece, or even the gap thickness between the rollers of a paper manufacturing machine. For reliability, malfunctions in the steady-state operation (transient operations and parameter fluctuations) must be controlled.
In the open-loop and closed-loop control of a converter having a two-storage system as a load circuit, there exist easily calculable relations between the excitation and the internal state variables of the system. Possible control processes, such as controlling the transistor conducting period, controlling the diode conducting period, controlling the capacitor voltage, and frequency control are known, for example, from the literature:
i. R. Oruganti, F. C. Lee Resonant Power Processors: Part II--Methods of Control, IEEE IAS Annual Meeting, Proceedings 1984, pp. 868-878 PA1 ii. R. Oruganti, F. C. Lee Resonant Power Processors: Part I--State Plane Analysis, IEEE IAS Annual Meeting, Proceedings 1984, pp. 860-867 PA1 iii. S. W. H. de Haan, H. Huisman Novel Operation and Control Modes for Series-Resonant Converters, IEEE Transactions On Industrial Electronics, vol. 32, no. 2, May 1985, pp. 150-157
Because of the complex relations and the high degree of complexity of the measuring and automatic control electronics, a working-point control is often undertaken by means of an open-loop or closed-loop control of frequency. In this case, often only a relevant state variable finds application as an input variable. If a parameter of the resonant circuit changes during operation, the operating ranges likewise shift. In the case of a frequency control which presupposes specific resonant-circuit parameters, the danger exists that there will be a departure from the established working point. In some instances, then, the point of maximum capacity can no longer be reached.
The paper, Solid State RF Generators for Induction Heating Applications, printed in IEEE IAS, 1982, Conference Record 1982, pp. 939-944, introduces a semiconductor inverter for induction heating. The inverter contains four power MOSFETs, which make up an H-bridge, to whose output terminals is linked a two-storage system as a load circuit. A controlling system is used to regulate the load current, two variable restrictions being provided. The one variable restriction compares the capacitor voltage to a limiting value. The voltage deviation is supplied via an integrator and a Schmitt trigger to the current-system deviation. The other variable restriction utilizes the phase-angle actual value between the load current and the excitation voltage of the inverter. This is compared to a phase-angle setpoint value. A phase-angle deviation is likewise supplied via a Schmitt trigger to the current-system deviation. The phase-angle actual value is determined, by means of an exclusive OR gate having a downstream smoothing element, on the basis of a measured load current and the control signals, in conjunction with a delay. It may be that this closed-loop control guarantees that there is never a departure from the operating range. However, the degree of complexity required for this control is considerable. Also, this control does not permit power to be controlled to a constant value in the case of a parameter fluctuation.